In recent years, a superconducting technology has been remarkably developed, and various types of examination apparatus such as a Magnetic Resonance Imaging (MRI) apparatus and a Nuclear Magnetic Resonance (NMR) apparatus have been used. In order to use the superconducting technology, a cryogenic temperature of 10 K or less, and further, 4 K or less is required to be realized. In order to realize such a cryogenic temperature, a refrigerator called as a cold head is used.
There are various types of cold head such as one of Gifford-McMahon type (GM type), one of Stirling type, and one of pulse type. In any one of the types of cold head, to achieve the cryogenic temperature, a regenerator material is used by being filled in a regenerator container called as a stage of the cold head. Depending on a design of the cold head, the regenerator container sometimes has one stage, and is sometimes divided into a plurality of stages such as two stages. Helium gas is passed through this regenerator container, and a specific heat per volume of the regenerator material is utilized to obtain the cryogenic temperature.
As the regenerator material, a rare earth compound, for example, is used. For example, by adjusting a particle size and an aspect ratio of a rare earth compound of Er3Ni, HoCu2, or the like, a strength and a filling factor are improved, to thereby improve a performance of cold head. Further, by improving the strength of the rare earth regenerator material, it is also possible to achieve a long-term reliability of 7000 hours.
Incidentally, there is an examination apparatus such as an MRI apparatus, as a part of pieces of examination apparatus, which is controlled by being continuously operated so that an examination of a patient can be conducted at any time. A guarantee time of a general cold head is said to be about 20000 hours to 30000 hours. This is mainly because of a deterioration of refrigerating capacity of a regenerator material due to a change over time, a clogging of a regenerator container caused by a breakage of the regenerator material under the continuous operation of the cold head.
A refrigerating capacity of a cold head is designed in accordance with a capacity of a regenerator material to be used. In accordance with the refrigerating capacity of the regenerator material, a combination of regenerator materials, an amount of using the regenerator material, an amount of supplying helium gas, and the like are adjusted. In a conventional regenerator material, a change over time or a breakage occurs after an elapse of time of about 20000 hours to 30000 hours, and accordingly, a long-term reliability of the aforementioned period of time or longer has not been obtained. Further, the cold head is designed to have a high airtightness to prevent helium gas from being leaked, so that a very large burden has been imposed on a maintenance for replacing the regenerator material.